PROJECT SUMMARY/ABSTRACT Fragile X Syndrome (FXS) is a common monogenic form of autism spectrum disorder (ASD). Symptoms of FXS include anxiety, intellectual disability, repetitive behaviors, social communication deficits, and abnormal sensory processing. Our previous studies have shown that elevated levels of Matrix Metalloproteinase-9 (MMP-9) contribute to the hyper-responsiveness of auditory cortex in Fmr1 KO mice by affecting perineuronal net (PNN) formation around parvalbumin (PV)-expressing inhibitory interneurons. Abnormal development of PV neurons most likely contribute to abnormal electroencephalography (EEG)-based phenotypes of auditory hypersensitivity in the Fmr1 KO mice that are remarkably similar to those seen in humans with FXS. We recently showed that embryonic deletion of Fmr1 in cortical excitatory neurons is sufficient to elicit cellular, electrophysiological, and behavioral phenotypes in Fmr1 KO mice. However, how the expression of Fmr1 in different cell types shapes normal cortical responses during circuit development is not known. Altered auditory responses in humans with FXS and Fmr1 KO animals suggest that abnormal development of the auditory circuits may underlie the deficits. PV/PNN deficits are first observed after the hearing onset at around postnatal day (P) 10 and hypersensitive neuronal circuits develop during third postnatal week (P14-P21 period). This period coincides with the critical period plasticity (CPP) window in the rodent auditory cortex, a postnatal window of structural and functional circuit development driven by sensory input. Synaptic and intrinsic properties of auditory cortex neurons also mature during this time window. Since the development of acoustic representations in primary auditory cortex is profoundly influenced by early experience, this project aims to: 1) determine whether deletion or rescue of Fmr1 in cortical excitatory neurons during the early postnatal period is sufficient to trigger or prevent the development of abnormal phenotypes in the auditory cortex of a mouse model of FXS, and 2) determine whether astrocyte-specific deletion of Fmr1 in developing or adult brain is sufficient to trigger abnormal phenotypes in the auditory cortex of a mouse model of FXS. This will be achieved through Cre-mediated deletion of Floxed Fmr1 gene in cortical excitatory neurons during P14-P21 period using CaMK2a (CreCaMk2a) promoter, or astrocytes using GFAP (ERT2-CreGFAP) promoter. My proposed research will use a multidisciplinary approach including electrophysiological (EEG), cellular, molecular, and behavioral methods to delineate how cell-specific deletion of Fmr1 during early postnatal period contributes to auditory hypersensitivity in FXS. Since these studies use clinically related techniques (i.e. EEG/MEA) to validate biomarkers relevant for neurodevelopmental disorders, this analysis will allow for a more direct comparison of animal-based research findings to human clinical studies.